Cisco calls dropped when recording is enabled

Symptoms

These symptoms are specific to recording enabled lines on Cisco central recording (RTP-forking with built-in bridge):

  • calls are dropped after transfer to a recorded line
  • ad-hoc conferences can not be initiated to 2 or more recorded lines
  • other call setup related issues

In a typical case, none of the above problems are present when only non-recorded lines are included in call scenarios.

Root cause

This is caused by codec changes during a recorded phone call.

The Verba Recording System supports G.711, G.722, G.729, iLBC and more (including codec changes), however the built-in bridge used for recording in Cisco phones has a limitation. When the phone starts a recording session using a certain codec, it gets "locked" into that codec.

During recording, an action that would otherwise require a simple codec change, will instead request a transcoder from UCM. If a transcoder can not be provided, the action will fail. Therefore if your UCM network is properly configured for transcoding between regions, this will not be a problem.

Resolution

This issue is a limitation of the phone system and not the recorder, therefore resolution is always done in the phone system.

The followings can be done by a Cisco UCM expert:

  1. Switch the codec selection to utilize e.g. G.711 everywhere
  2. Implement hardware transcoding between regions
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